....Yeah, I knew that the ratio isn't the same for Mega9 and DynaSys systems, but for some reason I thought that the shifters doing all the work about that ratio. I was wrong. thank You, your answer made me search a bit deeper for what I want

Got tons of miles both paved and trails including first race on this setup, never one complaint. RD shifts exact gear when I tell it to nice and smooth, never a hickup unless its my error (shifting under hard load, nasty mess on chain/cassette, etc and even then it shifts, just not as smooth and quick).

Just like Sram, Shimano model levels are somewhat interchangable. The shift ratio is in the shifters, not the derailleurs.

Well in same cases these guys may be right but if you can't use a 9s rd on 10s system then explain to me why I have perfect shifting using a 9s slx rd on '13 slx new style 10s shifters with hg62 10speed cassette???????

And to be more specific that exact m662 slx 9 speed rd that op is asking is exactly what I'm using.

So what's the problem, works perfect for me great shifting perfectly reliable???

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Because your version of perfect is far from it? Dyna-sys is in it's own bubble, you can't mix and match those derailiers and shifters with anything less than half ass results.